The present invention relates to the high fidelity duplexing, local transmission and reception of audio signals transmitted from sources such as stereos, televisions, or musical instruments.
The present invention relates to a system for local transmission of electromagnetic signals, such as television signals, audio signals, and the like to a remote receiver. Audio and television equipment for home use is becoming increasingly complex, involving multiple programming sources capable of reproducing sound and data with ever increasing fidelity. In the past speaker assemblies have been physically connected by cables to the receiver central receiver/player. Such arrangements are unaesthetic if the speaker is not located near the receiver/player because the wires connecting the speaker to the receiver/player may be visible. Installing wires under carpet or in the walls in order to hide them can be inconvenient and expensive. Also, wires can be unsafe for use in home entertainment systems in locations not proximate to the source of the acoustic data such as outdoors.
There have been innovations in the prior art using electromagnetic transmissions with low power to send data from the receiver/player to a receiver located at a speaker. The receiver sends the data to a speaker eliminating the need for the receiver/player and speaker to be connected by wire leads. Local low power signal transmission systems usually operate within a frequency band of 902 MHz to 928 MHz using Frequency Modulated (FM) signals. The use of traditional FM signals does not adequately reproduce the quality of sound that the receiver/player is capable of transmitting, thus making the user choose between high fidelity and convenience. The present invention overcomes this deficiency in the prior art.
Traditional stereophonic FM radio was developed from its predecessor, monaural FM radio. To insure compatibility, stereo FM radio was designed to be functional with the mono-FM radio transmission systems. Traditional wireless stereophonic transmission systems employ the architecture used in FM stereo systems because the component parts are easy to obtain and because they are produced in mass quantities, and are, therefore, inexpensive.
In current transmission systems, the right and left channels are mixed for FM modulation before transmission. The modulating signal spectrum of the prior art is shown in FIG. 1. In general, the prior art comprises a pair of amplifiers forming audio processing circuitry which provides selection between one mode of undistorted amplification, one mode of odd harmonic generation, and one mode of even harmonic generation. A double sideband suppressed carrier signal is formed from the output of an oscillator and the difference between the signals in the two channels and is combined with signals from the two channels and a pilot signal. This combination is employed to modulate a voltage controlled oscillator whose center frequency is the carrier frequency. The receiver includes an amplifier which receives the signal from its associated antenna, amplifies it to the limiting input level of a demodulator, and the stereo composite signal of the demodulator is decoded by an amplifier which provides two distinct channels of audio signals capable of driving a standard stereo amplifier. Regardless of design, the FM receiver must have sufficient bandwidth to pass the range of frequencies generated by the FM transmitter, and since the receiver should be super-heterodyne, to optimize sensitivity at the frequencies to which frequency modulation is restricted, the intermediate frequency bandwidth is an important factor in the design.
In most FM frequency ranges it is not always possible to obtain optimum performance at reasonable cost with a single-conversion super-heterodyne receiver. When adjacent channel selectivity is necessary, a low intermediate frequency channel is desirable; this, however lowers the image rejection ability of the receiver. If good image rejection is desired, a high intermediate frequency channel should be used, but this is not compatible with good adjacent-channel rejection unless an expensive intermediate frequency filter is employed. In many receiver designs, the high intermediate frequency channel is chosen so that a harmonic of the mixing oscillator used for the second mixer may be used with the first mixer to reduce the number of crystals in the receiver. In other cases, a frequency synthesizer is used to generate the proper mixing frequencies.
The third requirement of the FM receiver is a limiting device to eliminate amplitude variations before they reach the frequency detector. The simplest device for converting frequency variations to amplitude variations is an xe2x80x9coff-tunexe2x80x9d resonant circuit. With the carrier tuned a certain amount of RF voltage will be developed across the tuned circuit, and, as the frequency is varied either side of this frequency by the modulation, the RF voltage will increase and decrease in accordance with the modulation. If the voltage across the tuned circuit is applied to an ordinary detector, the detector output will vary in accordance with the modulation, the amplitude of the variation being proportional to the deviation of the signal, and the rate being equal to the modulation frequency. Only a small portion of the resonance curve is usable for linear conversion of frequency variations into amplitude variations, since the linear portion of the curve is rather short. Any frequency variation which exceeds the linear portion will cause distortion of the recovered audio. It is also obvious that an amplitude modulation receiver is vulnerable to signals on the peak of the resonance curve and also to signals on the other side of the resonance curve.
The prior art FM signal includes the sum (R+L) and difference (Rxe2x88x92L) of right and left audio channels in different frequency domains but complete separation of right and left audio channel is not feasible, resulting in inferior audio channels separation which deteriorates the stereo effect. Another drawback of the prior art is that the demodulation process at the receiver cannot completely eliminate the pilot tone and the residual noise of the pilot tone will distort the information in the signal. Also, the pilot signal consumes some power, decreasing the efficiency of the transmitter.
It is an object of this invention to provide a high fidelity wireless stereophonic transmission system having a plurality of audio channels, a first oscillator, a combiner electrically connected to said first oscillator and electrically connected to one of said plurality of said audio channels wherein said first oscillator provides a first carrier frequency upon which the data of said one of said audio channels is imposed forming a composite signal and a second oscillator electrically connected to said combiner wherein said second oscillator provides a second carrier frequency to said composite signal for electromagnetic transmission. It is a further object of the invention to receive the electromagnetic transmission at an antenna connected to a receiver to receive said transmitted composite signals. A mixer is electrically connected to the receiver for receiving the composite signals. A local oscillator is electrically connected to the mixer wherein the local oscillator generates an intermediate frequency signal wherein the intermediate frequency signal is sent to the mixer and mixed with the composite signal. A first demodulator is electrically connected to the mixer to receive the signal to demodulate the second carrier frequency from the composite signals and a second demodulator is electrically connected to the first demodulator to demodulate the first carrier frequency from the composite signals.
It is a further object of the present invention to provide a method of transmitting stereophonic signals by generating a first carrier frequency, combining this first carrier frequency with first data to form a composite signal, generating a second carrier frequency and combining the composite signal with said second data and multiplexing said second data and composite signal on said second carrier frequency. The second carrier frequency containing the composite signal is demodulated from the second carrier frequency from the second data. The first data is demodulated from the first carrier frequency.
It is a further object of the present invention to provide an apparatus for high fidelity wireless stereophonic transmission having a plurality of audio channels carrying data, a means for producing a first carrier frequency, a means for modulating data wherein the data of said one of said audio channel is imposed on said first carrier frequency forming a composite signal and a means for producing a second frequency electrically connected to a combiner wherein the second oscillator provides a second carrier frequency to the composite signal for electromagnetic transmission.
It is a further object of the invention to provide a means to receive the transmitted composite signals, a means for generating an intermediate frequency signal wherein the intermediate frequency signal is sent to a means for mixing the intermediate signal with the composite signal. It is a further object to provide a first means for demodulating electrically connected to the means for mixing to demodulate the second carrier frequency from the composite signals and a second means for demodulating electrically connected to the first means for demodulating to demodulate the first carrier frequency from the composite signal. It is also an object to provide a plurality of means for filtering electrically connected to said audio channel for pre-processing data in said plurality of audio channels, prior to said data entering said combiner, a means for detecting over-modulation, a transmitting means and a means for amplifying the composite signal prior to transmission through an antenna.